This invention relates to any form of projectile launcher or gun which utilizes gas, plasma, explosive, or any compressible material to drive or propel a projectile. In one embodiment, this invention relates to projectile accelerators such as airguns, hypervelocity guns, and high velocity projectile launchers in which it is desired or beneficial to obtain a projectile velocity that is greater than the local speed of sound of the driving gas or compressible substance.
Early documentation of compressible gas powered projectile propulsion devices, such as airguns, dates back to around the middle of the 16th century according to Traister, Robert J, 1981, All About Airguns, Tab Books Inc., Blueridge Summit, Pa. These ancient airguns were generally military devices used to fire projectiles in the .30 to .60 caliber size range. They were usually pneumatic, having a pressure cylinder that was manually pressurized. The basic principles used in gas powered guns have changed only slightly over the years.
Gas driven guns now have a wide variety of applications. Low velocity sport airguns are commonly used for target practice, gun training, and for hunting very small game. Airgun competition is now an Olympic sport. Airguns are also used in military and science labs for various purposes. The military uses airguns to launch some types of missiles which have vibration sensitive electronics inside according to Jones, M. C., 1986, xe2x80x9cShock Simulation and Testing in Weapons Development,xe2x80x9d The Journal of Environmental Sciences, September/October, Vol. 29, pp. 17-21. Gas powered guns are also used in various types of field weapons. Airguns typically operate with low vibration compared to explosive driven guns. Gas powered guns also are typically more controllable than explosive powered guns. Hypervelocity guns are similar to airguns, but usually use explosives and high temperature light gases which have a high speed of sound to achieve much higher velocities.
Airguns today are generally low powered and low velocity compared to guns which use explosives or high temperature light gases to drive the projectile. The most significant factor in the low velocity limitation of airguns and low temperature compressible gas powered guns is the speed of sound in the driving gas that propels the projectile. However, hypervelocity guns that use explosives, hot gasses, light gasses, plasma, and other gas like propellants are also limited by the same principle. For example, compressible gas equations for a gas in steady state, isentropic flow show that gas traveling from a high pressure reservoir at rest, to a low pressure reservoir, through a constant area or narrowing passageway, cannot exceed the velocity of the local speed of sound. The assumption of isentropic flow is common practice for many airgun designers. Using a hydraulic analogy of an airgun shows that even though an airgun is an unsteady device, the sonic limitation still applies.
Efforts to minimize the effect of the sonic velocity limitation have led to developments such as xe2x80x9clight gas gunsxe2x80x9d as disclosed in U.S. Pat. No. 3,186,304 and mentioned in U.S. Pat. No. 5,303,633. Light gasses, such as hydrogen, have a high speed of sound and increase the attainable sonic velocity. For example, the speed of sound in hydrogen is about 4 times faster than the speed of sound in air at the same temperature.
Raising the temperature of the driving gas is another way to reduce the effect of the sonic velocity limit. Raising the temperature of a gas raises the speed of sound in the gas. Several methods that raise the temperature of the propellant gas immediately before firing a gun are used today. For example, in U.S. Pat. No. 3,311,020 a conventional piston compresses the propellant gas immediately before firing the gun raising the temperature very high. In U.S. Pat. No. 3,465,638 an explosion compresses the driving gas chamber increasing the temperature of the driving gas and thus raises its speed of sound. Many similar methods of adding heat to the driving gas have been used to raise the speed of sound of the driving gas; however, they are all nonetheless limited by the sonic limitation.
Similarly, U.S. Pat. No. 4,658,699 discloses a wave gun that uses an explosive to propel a piston which compresses the driving gas chamber. Rapid acceleration of the piston creates shock waves ahead of the piston which raise the pressure and temperature of the driving gas. This gun is also mentioned in U.S. Pat. No. 5,303,633 which attempts to improve upon the above mentioned technology. Again, the gas upstream from the projectile is limited by the sonic limitation.
U.S. Pat. No. 5,303,633 discloses a xe2x80x9cshock compression jet gunxe2x80x9d that implements a shaped charge, compressible gas, and converging-diverging nozzle to drive a projectile through a barrel. The explosive shaped charge provides high pressure and temperature gas upon detonation. Then the high temperature and pressure exhaust gasses accelerate through a converging-diverging supersonic nozzle. Upon exiting the nozzle, the supersonic driving gasses, preceded by an abrupt normal shock, hit the projectile. The normal shock rebounds from the projectile leaving higher temperature and pressure subsonic gas immediately behind the projectile. This high temperature gas immediately behind the projectile remains limited to sonic velocities as the projectile travels through the barrel.
The above mentioned gun types are dynamic devices, and the sonic limitation as applied to them should be clarified. Upon firing a gas powered gun, local flow properties such as Mach number, temperature, and pressure will vary with time and position within the gun because firing a gun is an unsteady process. Under some circumstances, the projectile velocity could be greater than the local speed of sound of the gas immediately behind the projectile in the above mentioned gun types. For example, if the gas temperature behind the projectile decreases as the projectile travels, the local speed of sound in the gas behind the projectile may lower to a value that is less than the velocity of the projectile. However, in this example, the projectile never exceeds the maximum local speed of sound attained in the driving gas immediately behind the projectile along its pathway through the barrel. Therefore, without the use of some type of supersonic projectile barrel, as herein described, the projectile velocity cannot exceed the maximum transient sonic velocity of the driving gas behind the projectile.
In the case of the xe2x80x9cshock compression jet gunxe2x80x9d in U.S. Pat. No. 5,303,633, the velocity of the projectile is also limited by the local speed of sound immediately behind the projectile. Combustion gasses may attain a supersonic velocity after passing through the converging-diverging nozzle. However, supersonic explosion gasses hitting the projectile will cause a normal shock to rebound from the projectile. Once a normal shock rebounds from the projectile, the gas immediately behind the projectile is high temperature and pressure, but is subsonic, and travels the same velocity that the projectile travels. The temperature rise after the shock wave from the driving gas hits the projectile will increase the speed of sound in the gas immediately behind the projectile to higher values. This increased temperature raises the limiting speed of sound. However, this method is also limited by the speed of sound in the driving gas as mentioned and clarified above.
U.S. Pat. No. 4,590,842 discloses a xe2x80x9cMethod of and Apparatus for Accelerating a Projectilexe2x80x9d that places multiple supersonic plasma spray nozzles along the projectile barrel that spray supersonic plasma through the barrel wall and against the back of the projectile as it passes each nozzle. FIG. 2 in this patent depicts supersonic plasma spray from the nozzle impacting the back side of the moving projectile which causes the supersonic plasma to slow down and create shock waves. This patent explains that the barrel is designed to fit loosely around the projectile at locations where projectile velocity is high to minimize friction. This loose barrel to projectile fit allows high pressure plasma from the back of the projectile to escape into the region in front of the projectile through the annular gap between the barrel and projectile. Apertures, or vents, may be placed in the barrel wall downstream from a plasma spray nozzle to vent plasma gasses that accumulate in front of the projectile. In this design, the projectile may reach speeds that are greater than the speed of sound of the driving gas because of the multiple impacts of supersonic driving gas against the projectile accelerating the projectile in multiple stages along its path through the barrel.
The above patent, U.S. Pat. No. 4,590,842, discloses that the purpose and design of the apertures, or vents, is to remove high pressure plasma that had accumulated in front of the projectile. The patent never indicates or claims that the apertures are designed to vent or control the gas behind the projectile. The patent also recommends a preferred size of aperture having a cross sectional area equal to approximately twice the barrel, or projectile, cross sectional area.
With the exception of the design disclosed in U.S. Pat. No. 4,590,842, the problem in all the above types of guns which use any type of compressible gas to accelerate the projectile is that the attainable projectile velocity is restricted by the speed of sound in the driving gas or gasses behind the projectile. Projectile velocity in the design disclosed in U.S. Pat. No. 4,590,842 is not limited by the speed of sound in the driving gas because of its modular supersonic plasma jets that repeatedly impact the projectile as it travels along the length of the barrel. The present invention allows driving gas propelling the projectile and the projectile itself to reach supersonic velocities without the complexity and cost of methods such as the one disclosed in U.S. Pat. No. 4,590,842.
A purpose of the present invention is to eliminate the sonic velocity limitation of gas driven guns which propel a projectile through a barrel or tube, thereby increasing the attainable projectile velocity. Another purpose of the present invention is to provide a method of controlling the local Mach number of the driving gasses along their travel through any variety of projectile barrel or tube. The driving gasses can be pressurized gas, explosive combustion products, or any gas-like substance.
In one embodiment of the invention, a projectile is driven through the barrel of a gun by a driving gas. There is provided a process to drive the projectile at supersonic velocities with the driving gas. The driving gas is employed to push the projectile through a first portion of a barrel until the projectile reaches a speed equal to the speed of sound in the driving gas at a location immediately behind the projectile. Portions of the driving gas are then bled off from behind the projectile at locations spaced along a second portion of the barrel in a manner to further accelerate the projectile.
In another embodiment, a projectile is accelerated along a tube using compressed gas as the motive force until the projectile reaches a velocity equal to the speed of sound, based on the speed of sound in the compressed gas immediately behind the projectile. Portions of the compressed gas are then vented off through the wall of the tube from locations behind the projectile, thereby providing transverse expansion of the compressed gas and further acceleration of the projectile.
In another embodiment of the invention, there is provided apparatus which can be employed to carry out the just described processes.
In one manifestation, a nozzle projectile barrel comprises a barrel wall extending from a breech end to a muzzle end. The barrel wall defines a main projectile passageway to permit the passage of a projectile driven by a driving gas therethrough. The barrel wall has a plurality of longitudinally spaced apart transverse passageways positioned in a region of the barrel between a point where the driving gas immediately behind the projectile achieves local Mach 1 and the muzzle end of the barrel wall. Local gas mass flow from the main projectile passageway through the longitudinally spaced apart transverse passageways causes supersonic driving gas flow with respect to the speed of sound in the driving gas within the nozzle projectile barrel.
In another manifestation, the nozzle projectile barrel comprises a barrel wall which defines a main projectile passageway and a plurality of longitudinally spaced apart transverse passageways. The barrel wall extends from a breech end to a muzzle end. The main projectile passageway is for the passage of a projectile driven by a driving gas. The plurality of longitudinally spaced apart transverse passageways are positioned in a region of the barrel wall between a point where the driving gas immediately behind the projectile achieves local Mach 1 and the muzzle end of the barrel wall. The transverse passageways have a size restricting driving gas outflow from the main projectile passageway to a mass flow rate causing the driving gas behind the projectile to expand in a direction transverse to the projectile velocity at a rate causing the driving gas to accelerate to supersonic velocities for accelerating the projectile to supersonic speed relative to the speed of sound in the driving gas.
In a further manifestation, an apparatus comprises a barrel, a projectile and a driving gas pressure source. The barrel is formed by a barrel wall and has a breech end and a muzzle end. The barrel wall defines a passage extending from the breech end to the muzzle end. The projectile has a cross section which closely matches the cross section of the passage. The projectile is positioned in the passage in sealing engagement with the barrel wall. The driving gas pressure source is connected to the breech end of the barrel, and the driving gas pressure source has a sufficiently high pressure to accelerate the projectile to local Mach 1 in the barrel. The barrel wall has a solid region extending from the breech end toward the muzzle end and a porous region extending from the muzzle end toward the breech end. The porous region is sufficiently permeable to the driving gas so that transverse expansion of the driving gas through the wall of the tube permits the driving gas to achieve supersonic flow as if the driving gas had passed through a diverging nozzle.
The porous nozzle projectile barrel of the present invention offers the simplicity of conventional gun technology but eliminates the sonic driving gas limitation and thereby improves the performance of existing conventional guns. Further, the ease of modifying conventional guns of nearly any type to implement a porous nozzle projectile barrel provides easy incorporation into nearly all current gun applications. The passageways through the barrel wall have an additional benefit of allowing gas in front of the projectile to escape from the barrel interior through barrel wall passageways, thereby decreasing the gas pressure in front of the projectile and increasing the projectile velocity.
Furthermore, the porous nozzle projectile barrel may be used with any type of propellant that provides or produces pressurized gas to drive a projectile through a barrel. The propellant may include gun powder or other explosives commonly used in rifles today, or static or spring-compressed gas as is used in airguns. The propellant or driving gas source may also include plasma, chemical reaction products, or any other substance that has physical properties similar to a gas.